Semiconductor lasers typically are fabricated on a wafer by growing an appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an epitaxy structure having an active layer parallel to the substrate surface. The wafer is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser mirror facets typically are formed at the ends of the laser cavity by cleaving the semiconductor material along its crystalline structure to define edges, or ends, of the laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. For most semiconductor devices, however, the foregoing cleaving process is imprecise, for it relies on the location and angle of the crystalline planes of the semiconductor material. With some materials, for example, there may be cleave planes of approximately equal strength that are oriented at such acute angles to one another that minute perturbations occurring during cleaving can redirect a fracture interface from one cleave plane to another. Furthermore, the cleaving process creates fragile bars and miniscule chips that are awkward to handle during testing. In addition, mechanical cleaving tends to be incompatible with later processing of the individual chips, as would be needed to provide a monolithic integration of components on a chip, for example, since the wafer must physically be broken to obtain fully functional lasers, and once the wafer has been cleaved it typically is in small pieces so that conventional lithographical techniques cannot readily be used to further process the lasers.
The foregoing and other difficulties resulting from the use of cleaved facets led to the development of a process for forming the mirror facets of semiconductor lasers through etching. This process, which is described in U.S. Pat. No. 4,851,368, for example, also allows lasers to be monolithically integrated with other photonic devices on the same substrate. The process described in this patent was extended to provide a process for fabricating ridge lasers having etched facets, as disclosed in “Monolithic AlGaAs—GaAs Single Quantum-Well Ridge Lasers Fabricated with Dry-Etched Facets and Ridges”, A. Behfar-Rad and S. S. Wong, IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992, and further described in the above-mentioned U.S. patent application Ser. No. 11/356,203. However, it has been found that the depth of the ridge in such devices, and its consequent location with respect to the active region in the laser structure, must be precise if consistent results are to be obtained in the fabrication process, and controlling the dry etch process sufficiently to produce consistent ridge depths is very difficult.
Because controlling the ridge etch depth in a dry etch is difficult, etched facet ridge laser devices produced by prior processes have been found to have low single lateral mode yields and wide distributions in threshold current. In order to obtain as much as a 30-40% yield of usable devices with these processes it has been necessary to use a multiple-step etch procedure to prevent the ridge etch from extending too deeply into the epitaxial structure. This requires that the dry etch be repeated three or four times and the ridge etch depth measured after each etch in order to obtain the proper dimensions, and this effort is time consuming and significantly increases the cost of these devices.